1. Field of the Invention
The present invention relates to self-aligning fastening devices and means for installing same.
2. Description of the Prior Art
Since the advent of high performance commercial and military aircraft, the industry has faced the problem of providing rapid access to internally installed equipment for the purpose of inspection, service, maintenance and/or replacement. In doing this the nature of aircraft loads must be considered. The major loads imposed on the structure are those due to flight, landing and handling. In modern aircraft these are carried through the aircraft skin and, using the wing as an example, vary from low near the tip to high near the fuselage.
The constraints imposed on the designer include weight, cost, time and reliability. Historically, he has had to make a choice between two basic methods of handling the loads in the area of the cutout in the skin created by the need for internal access. He may elect to use a thin guage non-stressed panel in conjunction with added internal structure designed and located to carry the primary bending loads around the cutout. This usually takes the form of a machined forging. The removable access panel is light since it carries only local air loads. Oversize holes are used in both the panel and the internal structure. Together with a floating receptacle mounted on the internal structure, this design generally accepts misaligned holes and is usually used with a quick release type fastener. Tooling and labor costs for the panel installation are lower since only non-precision drill fixtures are required. An offset to this is the added cost and weight of the machined forging with its attendant tooling in the form of a forging die and precision milling and drill fixtures. This type of installation is the heavier.
The alternate method is to use an access panel, usually of the same thickness as the surrounding skin, with close fitting holes in both the panel and the internal structure and connected with close tolerance fasteners. These features provide the necessary bearing area and the intimate contact between all the load carrying members necessary to transfer the high shear and bending loads involved. This approach requires master tooling, precision drill fixtures and a high degree of labor skill. This type of installation is the lighter of the two methods and as such, in aircraft, is preferred.
The costs of both types of panel installation are initially about the same. However, when considered in terms of the "Life Cycle Costing" system used by the military in arriving at the total cost of the original acquisition and subsequent support and maintenance of an aircraft fleet over its expected life, usually twenty years, the costs of the stressed access panel become exorbitant. This can easily be explained.
Experience has shown that minute shifting or working of the aircraft structure, when subjected to flight/landing/handling loads, very easily uses up the slight clearance between close fitting fasteners and holes. As an example, using a 0.25 inch diameter screw, the total diametral clearance is ##EQU1## With fits like this, all too frequently when a panel is removed, the load is relieved and one or more of the fasteners cannot be replaced. The standard repair is to drill the holes out to accept the next larger size fastener. This can usually be done only once for a given hole before running out of acceptable edge distance for the fastener. When this occurs, the usual remedy is to requisition a replacement panel, preferably blank, and completing it on the spot to fit the structure. It is this condition that necessitates the procurement, storage and disbursement of a high percentage of spare access panels for the full life span of the aircraft fleet. It is readily apparent that this is not only expensive in terms of parts, storage facilities, labor and time, but also results in grounded aircraft, useless until they are repaired.
Prior special fastener art in this area, while called high shear fasteners, does not, in most cases, meet the intimate contact criteria referred to herein for high load carrying devices and none can be replaced with a larger version of the same.
The present invention eliminates the basic problem by permitting the original design to accept mating holes in both the access panel and the internal structure that are grossly misaligned either initially or subsequently while retaining intimate contact between all load carrying members.
Objects of the present invention are to:
a. Permit the creation of a high shear, bending, and local tension load attachment between two structural members whose attachment holes are either initially or subsequently grossly misaligned.
b. Permit the use of standard fastener hardware.
c. Achievement of a quick-release capability.
d. Permit fastener installation from one side when necessary.
e. Eliminate the need for master tooling and precision drill fixtures, using instead standard tolerance, low cost drill fixtures.
f. Reduce the personnel skill level required to make the original equipment and maintain it in service.
g. Reduce the man hours required to make the original equipment and maintain it in service.
h. In the case of aircraft, save the cost of forge dies, forgings and machining by using stressed removable panels.
i. In the case of aircraft, reduce the empty weight by using stressed removal panels and use this weight savings to increase any combination of range, payload and performance.
j. In the case of aircraft, eliminate, to a large degree, the need for procurement, storage and disbursement of spare access panels, with the attendant high costs, for the life of the aircraft fleet. This is normally fifteen to twenty-five years.
k. In the case of aircraft, decrease their down time and labor costs caused by removable panels that cannot be satisfactorily replaced without undue effort.
l. Permit the retention of the fastener and nested bushings in the removable panel when it is removed.
m. Provide the means by which a fastener can be installed through a flush surface bushing assembly under adverse conditions.
n. Provide the means by which the eccentrically located holes through a protruding bushing assembly can be brought into alignment for installation of a fastener.